1. (a) Field of the invention
The present invention relates to a method of forming amorphous silicon which is a semiconductor material for use in various kinds of semiconductor devices, and in particular to a method of forming amorphous silicon which is capable of providing good ohmic contact in case aluminum which is a most popular electrode metal when it is intended to produce an ohmic contact on a semiconductor device is used as an electrode for ohmic contact with amorphous silicon.
2. (b) Description of the prior art:
As is well known, amorphous silicon (hereinafter to be briefed as "a-Si") as a semiconductor material has recently been attracting the attention of the concerned as being a semiconductor material having a wide range of applications, being usable in various kinds of semiconductor devices or equipments, and its development is making a progress intensively. Its practical applications are also under way. The reasons for the high applicability of amorphous silicon may be as follows.
To begin with, the electrical characteristics of this a-Si can be varied widely by appropriately selecting its forming conditions, so that the degree of its semiconducting, insulating and photo-conducting properties can be controlled as desired. Since an a-Si is not a single crystal, there is no longer the need to consider such aspects as regularity of lattice or possible lattice mismatch relative to other semiconductor material, and this permits one to use a-Si mainly for the formation of thin films, which leads to enhancing the freedom not only in the designing of a semiconductor device but also in the size of the device, the device constitution and the device-fabricating process.
As such, there have already been developed various kinds of semiconductor devices and equipments employing an a-Si to serve as an active semiconductor material. Their typical examples include solar battery, image sensor, photo-sensor and thin film transistor. Also, consideration is being paid to the employment of an a-Si to serve as an insulator to form a protection film therewith. Especially, a solar battery using an a-Si is recently regarded to have a quite promising future in that the battery is a device manufactured at low cost and possesses a large active area and provides a high production yield.
This a-Si can be produced by any one of the various methods including vacuum evaporation, sputtering, reactive sputtering, and plasma chemical vapor deposition. Among these methods, the most typical method of its production is the so-called plasma chemical vapor deposition method which allows one to design, with a considerable freedom, the mechanical and/or electrical characteristics of a thin film of a-Si.
Hereunder will be described a method of forming an a-Si film by relying on the plasma chemical vapor deposition (hereinafter to be briefed as P-CVD) method, i.e. by decomposition of SiH.sub.4 gas by glow discharge with electric power of radio frequency (AC) or with electric power of direct current (DC).
One example of the apparatus for forming an a-Si film is shown in FIG. 1. The process of forming an a-Si film will be described by referring to FIG. 1. Numeral 1 represents a substrate such as a glass plate. Two plate electrodes 2 and 3 are positioned to face each other at a distance therebetween within a reaction chamber 4 which communicates a vacuum system 6. Said substrate 1 for use in the formation of an a-Si film is placed on one 2 of the plate electrodes 2 and 3. To this plate electrode 2, there may be additionally provided a heater 5 as required. In order to enhance the uniformity of the a-Si film which is to be produced, it is desirable to rotate the plate electrode 2 by a motor 11.
The apparatus includes a reaction chamber 4 and also includes a gas supply system which is provided separately from the reaction chamber 4 and is comprised of a gas chamber 9 containing SiH.sub.4 therein, another gas chamber 10 containing such impurity gas as PH.sub.3, and a gas supply controller 8 for controlling the flow rate and/or the ratio of gases to be mixed. The fluid such as air which may be present in the interior of the reaction chamber 4 is evacuated preliminarily by means of the vacuum system 6. Thereafter, a gas or a mixed gas is conducted from said gas supply system into the reaction chamber 4 through appropriate piping. Such gas is supplied uniformly onto the substrate 1 through a number of small perforations formed through the plate electrode 3 which is disposed to oppose the plate electrode 2 within the reaction chamber 4.
At the time a gas is supplied into the reaction chamber 4, an electric power is applied across the plate electrodes 2 and 3 from an external power supply 7, so that there is produced a glow discharge within the reaction chamber 4, with the result that SiH.sub.4 or a mixture of SiH.sub.4 and PH.sub.3 is decomposed, and an a-Si is deposited on the substrate 1.
The electric power which is applied to the apparatus may be a direct current (DC) or an alternate current (AC) of the order of radio frequency, or their superposition, depending on the desired property of the a-Si film which is to be produced.
The method mentioned above has many features such that the temperature of the substrate can be set low; that the forming process can be simplified in that, by substituting the impurity gases, a pn junction can be formed by the same forming steps; that the thickness of the a-Si film can be controlled as desired; and that a film having a large area can be easily obtained.
As stated earlier in this specification, the a-Si film which is thus obtained is considered to be very promising as a new semicondutor material, and its practical use has started in some limited fields. However, the controlling of such aspects as the growth mechanism, defects, content of hydrogen atoms, impurity and carrier mobility of an a-Si film still leaves many points that require elucidation and solution, and their analysis is demanded.
For example, the content of hydrogen atoms is considered to affect the characteristics of an a-Si film for the following reasons. It will be appreciated that an a-Si film produced from decomposition of SiH.sub.4 should contain hydrogen atoms. These hydrogen atoms will combine the dangling bond of silicon so that it will be able to reduce such density of localized level, i.e. the density of such dangling bond, which traps or releases carriers in an a-Si. As a result, it is considered to be possible to form a pn junction with an a-Si film. As such, the presence and the volume of these hydrogen atoms will largely contribute to the property of the a-Si film. Also, in case SiF.sub.4 is employed inplace of SiH.sub.4 for the formation of an a-Si film, it should be appreciated that fluorine atoms will be contained in the a-Si film produced. In the similar way as for SiH.sub.4, the presence and the volume of fluorine atoms will become problematical. However, it has been said that there is a difference between the influence of the hydrogen atoms and that of the fluorine atoms relative to the property of an a-Si film. Comparison therebetween has now become an important task of the concerned.
Now, in case a semiconductor device or equipment is constructed by using an a-Si film to serve as an active material, there arises the necessity for forming an electrode on such device by the employment of such metal as will allow the provision of an ohmic contact on the a-Si film.
The conditions which are required of an electrode metal for the provision of an ohmic contact on a semiconductor device include, as are well known, small resistivity, lack of rectifying property relative to a semiconductor material (absence of electrical potential barrier for carrier flow), lack of possibility to become an injection source of minority carriers, lack of capability to react against a semiconductor material, absence of migration of electrode metal at the surface or into the interior portion of a semiconductor, and easiness of handling.
Such demands as mentioned above have led to the consideration to use Pd or Pt having a relatively large work function for the formation of a p type a-Si film, though high in cost, and to use such metal as Al or Mo having a relatively small work function for the formation of an n type a-Si film.
Also, in case a transparent electrode is needed for the transmission of light rays, there are considered to use, for example, In.sub.2 O.sub.3, SnO.sub.2 and I.T.O. (Indium Tin Oxide).
It should be understood as a matter of course that a portion of the a-Si region which is brought into contact with such electrode as mentioned above is made into a highly-doped n.sup.+ type a-Si region or a p.sup.+ type a-Si region by the use of an impurity gas such as PH.sub.3 or B.sub.2 H.sub.6. Among those electrode materials mentioned above which are used for the provision of an ohmic contact, aluminum not only satisfies the abovesaid various requirements but also is a material having sufficient good results in the past in that: it allows one to form an electrode film by a simple technique; that it has a good adhering property; that it is a material available at low cost; that it allows an easy bonding of a lead wire; and that is has a high reliability. It is desirable to use aluminum for an a-Si film from the abovesaid aspect also, and thus various attempts have been made so far. However, aluminum has not yet been used as a material, in practice, to obtain a stable ohmic contact.
For example, by relying on a method and apparatus as shown in FIG. 1, there was formed a construction as shown in FIG. 2, i.e. an n type a-Si layer 12 is flanked at both sides thereof by two layers 13 and 13a for ohmic contact which are formed by a material such as Mo, Al and I.T.O. The I-V characteristic of the resulting device was measured, and the result is shown in FIG. 3.
As one of the forming conditions of a-Si film mentioned just above, the ratio between PH.sub.3 and SiH.sub.4 was PH.sub.3 /SiH.sub.4 =0.001. Although an a-Si film has a considerably low resistivity, the instance wherein an electrode metal was Mo and an instance wherein it was I.T.O. showed such characteristics that a good ohmic contact was obtained invariably. In case of aluminum as the electrode material, a non-ohmic contact characteristic, i.e. a rectifying characteristic, was exhibited. Furthermore, when the abovesaid ratio was increased to: PH.sub.3 /SiH.sub.4 =0.01 to provide a highly-doped n.sup.+ type a-Si, the result was that in case of aluminum as the electrode metal, a rectifying characteristic was exhibited in most cases as in the preceding example.
As discussed above, in spite of the fact that aluminum is a very desirable electrode material, it has yet been an unsatisfactory material for a-Si.